dual battery set up question

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Apr 9, 2015
I want to install a dual battery setup. The PO put the battery under the passenger front seat. And has every thing going to a ford type selinoid.. I have installed a aux fuse box also..

So my question is I have a battery dr also which will connect the 2 batteries.. do I route back to the battery under the seat or can I piggy back off of the aux fuse box ... I'm just kinda stumped on how to route this without getting compilated. Thanks for any help
Aux fuse box is hooked directly to starting battery as is the conections to the Ford selioind
Where are you going to mount the second battery?

What function/purpose will the second battery have? Winch? Cranking? Backup/extra power? Camping power?

I'm not quite understanding what you are using to connect the two batteries. A Blue Sea ACR is a much more efficient way than a Diode type isolator.
What's going to be the purpose of each of the two batteries? You need it clear in your head why you're adding a battery to a system that already has enough batteries to power all functions of your truck before you'll be able to know how it all should be wired up.

When I decided to go to a second battery it was so that I could have the ability to start my truck if I had run down my main battery by any combination of winching or not paying enough attention to my amp hour usage while sleeping ou and running stuff with the engine not running. I wanted to have a sort of onboard jump start that I knew would be there and charged up no matter what I did.

To get that I swiped the plan of one of Painless's kits - putting together my own kit of wire and pieces to duplicate the circuit of this: http://www.painlessperformance.com/Manuals/40102.pdf. So far it's worked fine as long as I remember to keep the second battery switched out unless needed. Painless now has a newer digital version of dual battery control that looks like it takes out the chance of my human error. I keep trying to remember to look into possibly changing my setup to do what their newer kit seems to do but...well, I forget to.
Thanks for all the info. I'll start think about alittle bit more
I have a Hellroaring dual battery isolator in both my FJ40 and Tacoma. Real life saver when I forget to turn off the 12 to 120 vac inverter and run down the main battery.

Hellroaring Battery Isolator/Combiner BIC-75300

Hellroaring has a ton of info on their site about the different philosophies of wiring up dual batteries to accomplish different things like @bikersmurf and @honk mentioned.

After reading everything on the Hellroaring site I decided that I would hook up my second battery as a backup battery with no permanently attached loads. There is a switch to force combine the two batteries to provide additional capacity for winching, or, as I have had to do several times, start the vehicle after I drain the main battery.

See this page for details:

Hellroaring Battery Isolator/Combiner notes for 4 Wheel Drive dual battery applications

No matter how you decide to hook up the batteries to your isolator make sure you use a large enough cable. I used #2 but #4 probably would have been adequate. Think how hot cheap small gauge jumper cables get after a short jumpstart - the amount of current your dead battery can draw off your good battery when you combine them may surprise you.

Good Luck!

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